Not to be absolutely certain is, I think, one of the essential things in rationality.

Bertrand Russell (1872 - 1970), "Am I An Atheist Or An Agnostic?", 1947
When one admits that nothing is certain one must, I think, also admit that some things are much more nearly certain than others.

Bertrand Russell (1872 - 1970), "Am I An Atheist Or An Agnostic?", 1947
I believe that uncertainty is really my spirit's way of whispering, "I'm in flux. I can't decide for you. Something is off-balance here."

Oprah Winfrey (1954 - ), O Magazine, June 2003

In these matters the only certainty is that nothing is certain.

Pliny the Elder (23 AD - 79 AD)
The only thing that makes life possible is permanent, intolerable uncertainty; not knowing what comes next.

Ursula K. LeGuin
Doubt is not a pleasant condition, but certainty is absurd.

Voltaire (1694 - 1778)